These three techniques are very similar in principle, and differ only in respect of when kill mud is pumped down.
In the Driller's method, the kill is split into two circulations. During the first, the kick fluid is circulated without changing the mud weight; once the kick is out, the mud is weighted up and pumped around the well on the second circulation.
The Wait and Weight method achieves both of these operations simultaneously. Kill mud is prepared before starting the kill, and the kick fluid is circulated out while this mud is circulated into the well.
In the Concurrent method, a compromise is adopted between these two methods. The kick fluid is circulated out while the mud being circulated in, is weighted up in stages, towards the kill weight.
Of the three methods, the Wait & Weight method is the most popular, and the one recommended for use in most instances. It produces lower well pressures than either of the other methods, with an evident benefit to safety. Each of the three methods is now described in detail.
1.3.1 The Driller's Method
In the Driller's method, the kick is circulated out of the hole using the existing mud weight. The mud weight is then raised to the required level and circulated around the well.
Two complete circulation's are thus required, as a minimum, for this method. Since it deals separately with the removal of the kick and the addition of kill weight mud, it is generally considered to be the simplest of well control methods, and it requires least arithmetic. However, this results, in the well being circulated under pressure for a relatively long time, possibly the longest of the three methods, with an increased possibility of choke problems. Also, the annular pressures produced during the first circulation are higher than produced with any other method.
CAUTION: BECAUSE VERY HIGH ANNULAR PRESSURE MAY ARISE WHEN KILLING A GAS KICK WITH THIS METHOD, CARE SHOULD BE TAKEN. ANNULAR PRESSURE WILL BE AT A MAXIMUM IMMEDIATELY BEFORE GAS ARRIVES AT SURFACE, AND CASING BURST PRESSURE LIMITATIONS MAY BE CRITICAL. FOR THIS REASON, THE DRILLER'S METHOD IS NOT GENERALLY RECOMMENDED.
In general, this method is most used on small land rigs where the Driller may have little help and limited equipment. A variant of this method is also used in some workover operations.
In addition the simplicity of the Driller's method makes it useful when only limited information is available about the well conditions.
2) If a slow circulating rate pressure, P SCR, has been taken, then calculate the pressure required on the drillpipe for the first circulation of the well.
Initial Circulation = Slow Circulation Rate + Shut In Drillpipe Pressure Pressure Pressure or:
KlllW(pigg)tMud = Old Mud Weight + [SIDPP (psi)* TVD(ft) * 0.052]
NOTE 1: This is a kill weight mud to balance formation pressure. It is the lowest possible mud weight which will 'kill' the well. Once the well is dead, it will be necessary to increase the mud weight further to provide a trip margin.
NOTE2: Some operators prefer to continue circulating the well while kill weight mud is being mixed. There is no theoretical reason why this should not be done, though it does result in further wear and tear on equipment under pressure - in particular the choke.
Once the kill mud reaches the bit, the pressure held on the drillpipe is just that required to circulate the mud around the well. This is the slow circulating rate pressure, increased slightly for the extra mud weight.
Final Circulating _ SlowCirculatirg Kill Mud Weight
The drillpipe pressure starts dropping below the initial circulating pressure, as the kill mud starts down the drillpipe, reaching the final circulating pressure when the kill mud reaches the bit. Thereafter the drillpipe pressure is held at the final circulating pressure by controlled opening of the choke, as the kill mud moves up the annulus.
A graph showing how drillpipe pressure drop from the initial to the final circulating pressure is shown in Figure 3 and this can be used as a guide to the drillpipe pressure required. The drillpipe pressure should drop according to the graph, as kill mud goes to the bit, without the choke being moved.
Figure 3 - Graph of Drillpipe Pressure as Kill Mud is Pumped
Figure 3 - Graph of Drillpipe Pressure as Kill Mud is Pumped
Because of the possibility that the annulus may not be circulated to completely clean during the first circulation, it may be preferable to work out how the drillpipe pressure should vary, as kill mud is pumped in and around the well. This will allow the drillpipe pressure to be used throughout, so eliminating the possibility of small gas bubbles in the annulus producing misleading information.
The following graphs depict the variations in pressure during the well circulation.
d) Determination of initial circulating pressure
If no slow circulating rate pressure has been taken, then the initial circulating pressure can be determined using the start-up procedures described in the circulations of the Driller's method.
Where the casing pressure has been held constant while the pumps are brought up to a kill rate, the drillpipe pressure reading will be the initial circulating pressure.
NOTE: This procedure is satisfactory at any time during a kill providing the mud weight in the drill string is not changing during the process. It is however preferable to maintain pump rate constant as much as possible. Any decision to change pump rate should be taken early.
The 'Wait and Weight' is sometimes referred to as the 'Engineers Method' or the 'One Circulation Method'. It does, at least in theory, kill the well in one circulation.
This is the preferred method used by most operators and recommended by many well killing experts. Its principal advantage is that it provides the lowest annular pressures during the circulation of the kill, making it the safest of the commonly used kill methods.
Once the well is shut in and pressures stabilised, the shut in drillpipe pressure is used to calculate the kill mud weight. Mud of the required weight is made up in the mud pits. When ready, kill mud is pumped down the drillpipe. At commencement enough drillpipe pressure must be held to circulate the mud, plus a reserve equivalent to the original shut in drillpipe pressure. This total steadily decreases as the mud goes down to the bit, until with kill mud at the bit, the required pressure is simply that needed to pump kill mud around the well.
The choke is adjusted to reduce drillpipe pressure while kill mud is pumped down the string.
With kill mud at the bit, the static head of mud in the drill pipe balances formation pressure. For the remainder of the circulation, as the influx is pumped to the surface, followed by drill pipe contents and the kill mud, the drillpipe pressure is held at the final circulating value by choke adjustment.
c) Procedure for the wait and weight method
The Wait and Weight method uses the same calculations already described for a drillpipe pressure schedule. The calculations are:
Kill Mud Weight = OngmalMud Wdght + [ ^ ^ ^ ^ (PPg) (PPg)
At the start of the circulation, with kill mud:
Initial Circulating SlowCirculatirg Rate Shut In Drillpipe Pressure _ Pressure + Pressure
Once the capacity of the drill string is calculated, it is possible to draw a graph showing how drillpipe pressure varies as kill mud is pumped down to the bit.
Once kill mud is ready, the start-up procedure is as previously described.
The choke is cracked open, the pump started to break circulation, and then brought up slowly to the Kill Rate.
While the Driller brings the pump up to the Kill Rate, the choke operator works the choke so as to keep the casing pressure at or as near as possible to the closed in casing pressure reading.
When the pump is up to the Kill Rate, the choke operator transfers to the drillpipe pressure gauge, adjusting the choke if necessary to achieve the INITIAL CIRCULATING PRESSURE on the drillpipe pressure gauge.
As the kill mud proceeds down the drillpipe, the drillpipe pressure is allowed to drop steadily from the Initial Circulating Pressure to the Final Circulating Pressure, by choke adjustment.
Where the kick is a small one, at or near the bottom of the hole, the drillpipe pressure tends to drop of its own accord as the kill mud moves down. Little or no choke adjustment is required.
Only in cases of diffused gas kicks with gas far up the annulus will significant choke adjustments be needed during this period.
After kill mud has reached the bit, the drillpipe pressures is maintained at the Final Circulating Pressure, until the kill mud returns to surface.
As with the Driller's method, this Final Circulating pressure is held constant as long as pump rate is held constant at the selected value. If, for any reason, the pump rate is felt to be wrong, it can be changed using the same procedure described previously. However, pump rate changes should be avoided, where possible.
While the pump rate is adjusted, the casing pressure is held steady by adjusting the choke. Once the pump is stabilised at its new speed, the revised circulating pressure is read from the drillpipe gauge. If a gas influx is very near to the surface, adjusting pump rate by holding a steady casing pressure may significantly increase the bottom hole pressure. This is due to the rapid expansion of gas near the surface. Alterations in pump rate are to be made early on!
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